According to a statement on the regulator's website, the FSA ordered RBS Securities Japan Limited to improve its compliance functions, which includes "clarifying the responsibility of the management and staff regarding [rate manipulation] and take preventive measures against recurrence of [Libor rigging]."

The FSA has also forced the unit to submit a report to the regulator by May this year, and every three months thereafter, in order to prove it is planning and implementing a change to its compliance procedures.

When RBS Securities Japan Limited pleaded guilty to one criminal charge of wire fraud, the bank entered what's called a deferred prosecution agreement under which it cooperates with the US Department of Justice, pay a $150m fine and have that charge and another antitrust charge deferred.